THE INFLUENCE OF THE RESPIRATORY MOVEMENTS UPON THE BLOOD-PRESSURE IN THE PULMONARY SYSTEM

¹ The Department of Physiology, Edinburgh University

Every inspiratory movement is accompanied by a fall in pulmonary blood-pressure, and every expiratory movement by a rise. These effects are purely mechanical, and are due to the increase in calibre of the pulmonary capillaries by inspiration and to their diminution by expiration.

The changes of pressure in the aortic system which are caused by the respiratory movements are dependent upon those in the pulmonary system, but bear no constant ratio to them, since differences are produced by changes in the rate of respiration and by other conditions affecting the rate of flow of blood in the pulmonary system. Most frequently, however, the rise of aortic pressure is synchronous or nearly so with the inspiratory fall in pulmonary pressure, and a fall of aortic pressure synchronous with the expiratory rise.

In the first stage of asphyxia, when the inspiratory movements are increased in force and the expiratory diminished, the pulmonary pressure falls, whilst the aortic pressure rises; whereas in the second stage, when the expiratory movements are accentuated, the pulmonary pressure rises. The aortic pressure may show a corresponding fall, but in many cases continues to rise owing to the effect of the accumulating CO₂ in the blood upon the vaso-motor centre.